The present invention relates to a method of producing visible, preferably transparent or translucent, continuous streaks and/or delimited fields in paper in conjunction with forming a paper web in a paper machine. The invention also relates to valuable paper, particularly banknotes, produced from such paper. More specifically, the invention relates to the manufacture of a continuous paper web containing delimited fields and/or streaks of varying size and in varying positions, and to valuable paper produced from such webs. The fields and/or streaks differ from the surrounding paper with regard to material, colour, opacity and/or transparency or with regard to some other readily identified property, preferably an optical property. The incorporated fields and/or streaks are preferably transparent or translucent.
The rapid development of reproduction methods and of fourcolour copiers in particular, places increasingly higher demands on the manufacturers of security paper, cheque paper and banknotes. These demands have led to the development of new methods, materials and products which are intended to prevent forgery or at least make successful forgery more difficult to achieve. Many of the methods proposed hitherto are complicated and expensive in practice and the products produced by these methods are encumbered with drawbacks, such as unsatisfactory durability.
There is thus a need for methods which will enable security paper that is difficult to forge by reproduction with the aid of colour copying and which can be printed in conventional printing presses with conventional printing inks intended for security print to be produced in a relatively simple and inexpensive manner.